nfsd4: minor NFSv2/v3 write decoding cleanup

commit db44bac41bbfc0c0d9dd943092d8bded3c9db19b upstream.

Use a couple shortcuts that will simplify a following bugfix.

(Minor backporting required to account for a change from f34b9568
"The NFSv2/NFSv3 server does not handle zero length WRITE requests
correctly".)

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>